1. Field of the Invention
The present invention relates to separation membrane modules for separating a specific component from a mixture containing a plurality of components. The present invention also relates to fuel vapor processing apparatus using the separation membrane modules.
2. Description of the Related Art
There has been applied to various fields of industry separation membranes capable of separating and recovering a specific component from a mixed gas containing a plurality of components mixed together or from a liquid in which a gas component is dissolved by utilizing a difference in dissolution or diffusion coefficient, etc. with respect to a polymer membrane. In general, a separation membrane has a structure M which a separation layer is layered on a porous support body, and a plurality of separation Membranes are arranged into a hollow case to form a separation membrane module. The hollow case has an introduction port through which an object of processing is introduced, a recovery port through which a specific component separated by a hollow fiber membrane is recovered, and a discharge port through which the residual component not transmitted through the hollow fiber membrane is discharged. For example, there is a hollow fiber membrane in which a separation layer is stacked on the inner surface or outer surface of a hollow fiber as a support body. And, as examples of a technique in which an improvement in terms of separation performance is achieved by the hollow fiber membrane, Japanese Laid-Open Patent Publication Nos. 5-184812, 2000-93729, 7-96152 and 10-296005 are known.
In the hollow fiber membrane module disclosed in the Publication Nos. 5-184812 and 2000-93729, there is used an inner pressure type separation membrane in which the object of processing is introduced into a hollow fiber membrane to separate a specific component through transmission to the exterior of the hollow fiber membrane via a separation layer; a conventional elongated hollow fiber membrane is divided into a plurality of hollow fiber membranes, which are connected in series, and a turbulent flow is generated at each connecting portion while reducing a pressure on the outside of each hollow fiber membrane, thereby achieving an improvement in terms of separation performance. In the hollow fiber membrane module disclosed in the Publication No. 7-96152, there is used an outer pressure type separation membrane in which a specific component is separated through transmission to the interior of a hollow fiber membrane via a separation layer from the exterior of the hollow fiber membrane; the inner diameter of the hollow fiber is gradually changed, or the thickness of the separation layer is gradually changed from a closed one end toward the other end (terminal end) serving as an outlet, whereby reduction in difference in specific component transmission amount per unit area is achieved, thereby achieving an improvement in terms of separation performance. More specifically, the inner diameter is gradually increased while keeping the membrane thickness uniform, or the membrane thickness is gradually increased while keeping the inner diameter uniform, or the membrane thickness is gradually increased while gradually reducing the inner diameter. In the case of an inner pressure type separation membrane, the inner diameter is gradually reduced from both ends toward the central portion in the longitudinal direction. In the Publication No. 10-296005, two or more kinds of hollow fiber membrane modules differing in gas transmission characteristic are connected in series, and a first stage degassing is effected through reduction in pressure in a first separation membrane module of higher oxygen transmission speed at a pressure higher than that of steam; then, the resultant degassed water is subjected to pressure reduction degassing at a pressure lower than that of steam in a second separation membrane module of low oxygen transmission speed and low steam transmission speed, thereby producing ultra-degassed water with high efficiency by using a vacuum pump of small capacity.
On the other hand, there exists an fuel vapor processing apparatus in which fuel vapor obtained through evaporation of gasoline stored in a fuel tank is temporarily retained through adsorption by a canister so that it may not be dissipated into the atmosphere and in which the fuel vapor is separated from the canister to be recovered in the fuel tank; to enhance the recovery efficiency, a separation membrane module is incorporated into this fuel vapor processing apparatus. Examples of such a fuel vapor processing apparatus are disclosed in Publication Nos. 2004-50042 and 2004-324488. In the Publication No. 2004-50042, reduction in pressure is effected in a canister by a pneumatic pump, and a gas containing separated fuel vapor is sent under pressure to a separation membrane module, whereby the fuel vapor, which is the specific component, is recovered in the fuel tank through condensation. The technique disclosed in the Publication No. 2004-324488 is an improvement over the Publication No. 2004-50042 to process fuel vapor with still higher efficiency, and recovery is performed while effecting separation at a plurality of stages by using a plurality of separation membrane modules. In the Publication Nos. 2004-50042 and 2004-324488, a flat separation membrane is used.
In the known art, for separating a specific component through transmission with high efficiency by an inner pressure type hollow fiber membrane, there is a general tendency to make the hollow fiber membrane relatively long in order to increase the length of time that the component is held in contact with the hollow fiber membrane. The lower the concentration of the specific component on the transmission side (the space outside the hollow fiber membrane), the easier it is for the specific component to be transmitted through the separation layer. However, when the hollow fiber membrane is long, the distance from the end portion of the hollow case accommodating it to the recovery port for the specific component becomes inevitably long. Thus, the specific component transmitted at the start end (inlet) portion and the terminal end (outlet) portion of the hollow fiber membrane is not easily discharged through the recovery port; the farther from the recovery port, the higher the concentration of the specific component in the transmission side space, resulting in reduction in separation efficiency. Further, there is a possibility that the specific component transmitted at the start end portion of the hollow fiber membrane is diffused to the terminal end (outlet) portion beyond the recovery port.
In this regard, in the Publication Nos. 5-184812 and 2000-93729, the hollow fiber membranes are connected in series, so that it is possible to generally increase the length of time that the specific component is held in contact with the hollow fiber membranes without making each hollow fiber membrane long. Further, at each connecting portion, it is possible to prevent diffusion of the specific component from the inlet side to the outlet side of the module. In this case, however, the conditions (atmosphere) acting on the hollow fiber membranes at different stages are different from each other. More specifically, the more downstream, the lower the concentration, flow rate, etc. of the specific component in the object of processing, with the separation efficiency being reduced gradually. However, in the Publication Nos. 5-184812 and 2000-93729, the same hollow fiber membrane is used and the specific component is transmitted under the same condition for the different stages, and therefore, the efficiency is low.
On the other hand, in the Publication No. 7-96152, the difference in specific component transmission amount per unit area is reduced by gradually changing the inner diameter, thickness, etc. of the hollow fiber membranes, so that the above problem in the Publication Nos. 5-184812 and 2000-93729 can be mitigated. It should be noted, however, that, in the Publication No. 7-96152, the inner diameter, thickness, etc. are only varied in a single hollow fiber membrane, and, in order to increase the length of time that the specific component is held in contact with the hollow fiber membrane, there is nothing for it but to increase the length of the membrane. That would involve the same problem as mentioned above. Further, in the Publication No. 7-96152, use of an outer pressure type separation mode is a prerequisite, and the inner diameter, etc. are gradually changed from one end to the other end; and thus, this idea cannot be applied as it is to an inner pressure type separation configuration. In fact, in the case of the inner pressure type separation configuration, variation is effected from both ends toward the central portion in the longitudinal direction; thus, it is impossible to solve the problem that the more downstream, the lower the concentration, flow rate, etc. of the specific component in the object of processing.
In the Publication No. 10-296005, the reduction in pressure is effected to a great degree on the upstream transmission side, and the reduction in pressure is effected on the downstream side to a smaller degree than on the upstream side. However, in the Publication No. 10-296005, separation membrane modules of different kinds are simply connected together, and the pressure reduction force is set accordingly, which means the pressure reduction force is not set taking into account the problems inherent in the configuration of the hollow fiber membranes and changes in the thickness thereof.
In the fuel vapor processing apparatus of the Publication No. 2004-50042, there is provided only one flat separation membrane module, so that the fuel vapor recovery efficiency of the apparatus cannot be regarded as high. On the other hand, in the fuel vapor processing apparatus of the Publication No. 2004-324488, a plurality of separation membrane modules are provided, and fuel vapor is separated at a plurality of stages, so that an improvement in terms of recovery efficiency is attained. However, providing a plurality of separation membrane modules leads to increase in the size of the apparatus. Further, this also involves a rather complicated piping route, which also leads to increase in the size of the apparatus.
Therefore, there is a need in the art for a separation membrane module that can effectively separate a specific component from a mixture of a plurality of components.